The two microelectrode voltage clamp technique is used to examine the relations between membrane voltage, ionic currents, and contraction in sheep cardiac Purkinje fibers. The principal goal of the project is analysis of the slow inward current (Isi) and its relation to contraction. Isi appears to be directly linked to contraction, and slowly conducted responses arising from the Isi system have been suggested as a cause of arrhythmias. Little is known about Isi in Purkinje fibers, because other currents mask its contribution to the net ionic current. The largest of the overlapping currents, the early outward current, is blocked by 0.5 mM 4-aminopyridine and another competing current is reduced to insignificance by low chloride bathing solutions. The remaining current resembles the Isi described in ventricular muscle. Experiments are planned to determine whether the current remaining after these treatments is indeed Isi, to determine the kinetics of Isi, and to determine the relation of Isi to contraction.